In the mobile communication system shown in FIG. 1, a core network 103 is connected with a radio network controller 1041 located in an access network 104 via a STM-1(level-1 Synchronous Transmission Module)/E1 (European Transmission Standard-E1)/T1 (North American or Japanese Transmission Standard-T1) line. Generally, a dedicated clock board (not shown in FIG. 1) is provided inside of the radio network controller 1041 for extracting a transmission clock from the STM-1/E1/T1 line, and then transmitting the transmission clock to backplanes of chassis at various layers. Various types of processing boards are connected to the backplanes of the chassis. The radio network controller 1041 is jointly composed by the chassis, clock boards, various types of processing boards and some other components. Each processing board in the radio network controller 1041 obtains the aforesaid transmission clock from the backplane of the chassis and generates a radio clock, that is, radio frame numbers, derived from the transmission clock independently according to its own requirements.
The types of the boards in the radio network controller is actually increased by the above method for extracting the transmission clock with a dedicated clock board, and thus it reduces the reliability of the mobile communication system. Moreover, additional cables are generally needed in this method to connect external STM-1/E1/T1 lines with the clock board, which increases the complexity of the external lines of the radio network controller. Furthermore, in order to make the various types of processing boards generate radio clocks based on the transmission clock, each processing board has to be designed individually. Consequently, no ready-made commercial board can be used in the system employing this method, and thus it reduces the flexibility of system development and increases development costs.